In a hydrogen infrastructure improvement business for building a hydrogen society, it is important to spread hydrogen stations for storing and supplying high-pressure hydrogen. In order to configure the hydrogen stations having high reliability, development of high-pressure hydrogen gas pressure vessels is indispensable, and development of excellent materials for the pressure vessels has been desired. Here, metal materials, particularly steel materials, show promise as the materials for the pressure vessels, from the viewpoints of cost and recyclability.
As a technical trend, it has been desired that pressure of stored gas is made higher in order to extend a travel distance of hydrogen cars, and it has been envisioned that the high-pressure hydrogen gas of 35 MPa or more is stored in the pressure vessels of the hydrogen stations. However, in conventional carbon steels or high-strength low-alloy steels, it has been conceivable that hydrogen environment embrittlement occurs under a high-pressure hydrogen gas environment. Thus, a steel material, which can be used under a high-pressure hydrogen gas environment of 35 MPa or more, has been almost limited to an austenitic stainless steel until now. The austenitic stainless steel is generally more expensive than a low-alloy steel. Further, the austenitic stainless steel has a stable austenite phase up to room temperature, so that strength adjustment by heat treatment cannot be performed. Accordingly, a high-strength low-alloy steel has been desired as the material for the pressure vessels for storing the higher-pressure hydrogen gas.
In order to meet such requests, there have been proposed a carbon steel or a low-alloy steel under a high-pressure hydrogen environment, a seamless steel pipe produced therefrom, and a method for producing the same (for example, Patent Literature 1). The steel proposed in the Patent Literature 1 decreases an amount of diffusible hydrogen in the steel by controlling the Ca/S ratio of components in order to improve high-pressure hydrogen environment embrittlement resistance characteristics.